11 research outputs found
Lithographic Tuning of Polymeric Thin Film Surfaces by Stress Relaxation
We report a facile soft lithography (SL) technique that
allows
fabrication of patterned polymeric surfaces with feature height varying
between 0 and <i>h</i><sub>0</sub>, using a single stamp.
The method relies on the partial relaxation of the applied stress
in a viscoelastic polymer thin film imprinted under a transient external
load using a flexible stamp. The applicability of the technique is
demonstrated for thermosetting (rubbery) as well as thermoplastic
(glassy) polymers over a wide range of lateral dimensions. The lateral
dimension and the periodicity of imprinted patterns remain identical
to that of the original stamp. The method has potential applications
in creating templates for performing combinatorial experiments related
to wetting and dewetting studies, adhesion, nanotribology, microfluidics,
etc
Solvent-Vapor-Assisted Dewetting of Prepatterned Thin Polymer Films: Control of Morphology, Order, and Pattern Miniaturization
Ultrathin (<100 nm) unstable polymer
films exposed to a solvent
vapor dewet by the growth of surface instability, the wavelength (λ)
of which depends on the film thickness (<i>h</i><sub>f</sub>). While the dewetting of a flat polymer thin film results in random
structures, we show that the dewetting of a prepatterned film results
in myriad ordered mesoscale morphologies under specific conditions.
Such a film undergoes rupture over the thinnest parts when the initial
local thickness of these zones (<i>h</i><sub>rm</sub>) is
lower than a limiting thickness <i>h</i><sub>lim</sub> ≈
10 nm. Additionally, the width of the pattern grooves (<i>l</i><sub>s</sub>) must be wider than λ<sub>s</sub> corresponding
to a flat film having a thickness of <i>h</i><sub>rm</sub> for pattern-directed dewetting to take place over surface-tension-induced
flattening. We first present an experimentally obtained morphology
phase diagram that captures the conditions where a transition from
surface-tension-induced flattening to pattern-directed-rupture takes
place. Subsequently, we show the versatility of this technique in
achieving a variety of aligned mesopatterns starting from a prepatterned
film with simple grating geometry. The morphology of the evolving
patterns depends on several parameters such as the initial film thickness
(<i>h</i><sub>f</sub>), prepattern amplitude (<i>h</i><sub>st</sub>), duration of solvent vapor exposure (SVE), and wettability
of the stamp used for patterning. Periodic rupture of the film at
regular intervals imposes directionality on the evolving patterns,
resulting in isolated long threads/cylindrical ridges of polymers,
which subsequently disintegrate into an aligned array of droplets
due to Rayleigh–Plateau instability under specific conditions.
Other patterns such as a double periodic array of droplets and an
array of holes are also possible to obtain. The evolution can be interrupted
at any intermediate stage by terminating the solvent vapor annealing,
allowing the creation of pattern morphology on demand. The created
patterns are significantly miniaturized in size as compared to features
obtained from dewetting a flat film with the same <i>h</i><sub>f</sub>
Thermally Tailored Gradient Topography Surface on Elastomeric Thin Films
We
report a simple method for creating a nanopatterned surface with continuous
variation in feature height on an elastomeric thin film. The technique
is based on imprinting the surface of a film of thermo-curable elastomer
(Sylgard 184), which has continuous variation in cross-linking density
introduced by means of differential heating. This results in variation
of viscoelasticity across the length of the surface and the film exhibits
differential partial relaxation after imprinting with a flexible stamp
and subjecting it to an externally applied stress for a transient
duration. An intrinsic perfect negative replica of the stamp pattern
is initially created over the entire film surface as long as the external
force remains active. After the external force is withdrawn, there
is partial relaxation of the applied stresses, which is manifested
as reduction in amplitude of the imprinted features. Due to the spatial
viscoelasticity gradient, the extent of stress relaxation induced
feature height reduction varies across the length of the film (<i>L</i>), resulting in a surface with a gradient topography with
progressively varying feature heights (<i>h</i><sub>F</sub>). The steepness of the gradient can be controlled by varying the
temperature gradient as well as the duration of precuring of the film
prior to imprinting. The method has also been utilized for fabricating
wettability gradient surfaces using a high aspect ratio biomimetic
stamp. The use of a flexible stamp allows the technique to be extended
for creating a gradient topography on nonplanar surfaces as well.
We also show that the gradient surfaces with regular structures can
be used in combinatorial studies related to pattern directed dewetting
Creation of self-organized complex meso patterns in sol-gel thin films by confined capillary dynamics
We report a simple and facile technique for creating complex mesoscale patterns with two-dimensional (2-D) orders on the surface of a sol gel derived silica thin films using a combination of a topographically patterned stamp and a substrate, each having a one-dimensional (1-D) grating structure (stripes). The self-organized pattern formation results as a consequence of capillary flow of the sol film in liquid state under a confining stamp. The key novel aspect of the work lies in using a topographically patterned substrate, dip coating on which results in a film with an undulating top surface. When a grating stamp is brought in conformal contact with such a film in a manner so that the direction of the stamp stripes are at right angles to the direction of the undulations on the film surface, a periodically ordered spatial variation in the extent of contact between the film and the stamp results. Subsequent capillary flow of the liquid sol along the stamp walls occurs only at the locations where the stamp surface and the film are in direct contact, resulting in an array of pillars with 2-D order. We further show that the morphology of the final patterns can be tailored by varying various parameters such as the initial film thickness, wettability of the stamp, and periodicity of the substrate. We further discuss the gradual oxidation of the sol film with progressive thermal annealing and the associated shrinkage of the structures in terms of pattern dimensions. Generation of meso patterns with 2-D order on an inorganic sol gel film surface using a stamp and a substrate, each having strictly 1-D features, is the key novel aspect of the work, as it is a rare example when patterns other than a mere negative replica of a stamp are created on a sol gel thin film by any soft lithography technique
Reversible morphological switching and deformation hysteresis in electric field mediated instability of thin elastic films
Capillary Force Lithography Pattern-Directed Self-Assembly (CFL-PDSA) of Phase-Separating Polymer Blend Thin Films
We
report capillary force lithography pattern-directed self-assembly
(CFL-PDSA), a facile technique for patterning immiscible polymer blend
films of polystyrene (PS)/polyÂ(methyl methacrylate) (PMMA), resulting
in a highly ordered phase-separated morphology. The pattern replication
is achieved by capillary force lithography (CFL), by annealing the film beyond
the glass transition temperature of both the constituent polymers,
while confining it between a patterned cross-linked polyÂ(dimethyl
siloxane) (PDMS) stamp and the silicon substrate. As the pattern replication
takes place because of rise of the polymer meniscus along the confining
stamp walls, higher affinity of PMMA toward the oxide-coated silicon
substrate and of PS toward cross-linked PDMS leads to well-controlled
vertically patterned phase separation of the two constituent polymers
during thermal annealing. Although a perfect negative replica of the
stamp pattern is obtained in all cases, the phase-separated morphology
of the films under pattern confinement is strongly influenced by the
blend composition and annealing time. The phase-separated domains
coarsen with time because of migration of the two components into
specific areas, PS into an elevated mesa region and PMMA toward the
substrate, because of preferential wetting. We show that a well-controlled,
phase-separated morphology is achieved when the blend ratio matches
the volume ratio of the elevated region to the base region in the
patterned films. The proposed top-down imprint patterning of blends
can be easily made roll-to-roll-compatible for industrial adoption
Differential Behavior of Normal and Fibrotic Fibroblasts under the Synergistic Influence of Micropillar Topography and the Rigidity of Honey/Silk-Fibroin Substrates
We
report differential proliferation behavior of normal and fibrosis
associated human oral fibroblasts on micropillar honey embedded silk
fibroin substrates (HSF). Oral fibroblasts of different origins manifest
differences in proliferation rate, morphology, and the cytoskeletal
arrangement on HSF substrates with distinct topography (H, D, and
S), stiffness, and honey concentration. It is observed that the proliferation
rate is maximized for normal and inhibited for fibrosis associated
fibroblasts on a HSF substrate surface with moderate height of ∼8.5
μm and 2% honey concentration. Molecular expression analysis
reveals decrease in c-myc and p53 expression in later cells validating
the inhibition of their proliferation rate, which is further correlated
with the decreased Col I and Col III expression on this substrate.
A substrate with enhanced interspacing and intermediate mechanical
stiffness (0.57 ± 0.32 μN/nm) favors strong adhesion and
stable cell–matrix interaction for normal cells, while exhibiting
negative influence on fibrotic fibroblasts with poor adhesion and
spreading capability. Decrease in vimentin, fibronectin expression,
and cytoskeleton reorganization justify the poor stability of later
cells on the optimized substrate, thereby allowing selective modulation
of normal and fibrosis associated fibroblasts under the synergistic
influence of honey concentration, topography, and rigidity of HSF
substrates. The work highlights the possible therapeutic efficacy
of honey based micropatterned substrates as smart patches for fast
wound healing and in minimizing the chances of recurrence of precancer
post oral tumor resection surgeries